The present invention relates to a pulsewidth modulation apparatus, applied to an electrophotography system utilizing exposure means, such as a laser beam or the like, an image printing apparatus using the pulsewidth modulation apparatus, and a control method thereof.
A construction of a general laser-beam printer is shown in FIG. 11, and operation thereof is described.
A laser chip 23 comprises a laser diode and a photodiode which receives backlight of the laser diode. An LD driver 24 supplies a driving current Id for controlling emission of the laser diode. A monitor current Im, detecting the amount of light emitted from the photodiode, is inputted to the LD driver 24 to perform automatic power control (APC) of the amount of light emission of the laser diode.
A modulated laser beam generated by the laser chip 23 is polarized by a polygon mirror 18, which is fixed to a motor axle and rotated in the direction of the arrow shown in FIG. 11, and scanned on a photosensitive drum 20. An f-xcex8 lens 19 is provided to condense the polarized modulated laser beam on the photosensitive drum 20 at a regular linear velocity. The photosensitive drum 20 and printing toner are electrostatically charged to predetermined levels in advance. Since the amount of toner attachment changes in accordance with the amount of light irradiated on the photosensitive drum 20, a halftone image can be printed. A BD mirror 21 is provided with a fixed mechanical position relation with the photosensitive drum 20. A laser beam reflected by the BD mirror 21 is inputted to a photoreceptive diode 22, and used to detect a data write starting position on the photosensitive drum 20. An output of the photoreceptive diode 22 is inputted to a horizontal synchronization signal generator 27 to generate a horizontal synchronization signal BD. The signal BD is inputted to a pixel modulator 25. The pixel modulator 25 generates a pixel clock synchronizing with the horizontal synchronization signal BD or a coefficient-fold clock of the signal BD. Based on the pixel clock, a read clock RK for reading pixel data is inputted to a pixel data generator 26. The pixel data generator 26 outputs pixel data D and a write clock WK to the pixel modulator 25. Based on the inputted pixel data, a pixel modulation signal ON, which enables modulation of a desired amount of a laser beam, is outputted to the LD driver 24.
Furthermore, the above-described construction may be provided for four photosensitive drums 1020a to 1020d as shown in FIG. 12. By providing each of the photosensitive drums with the structures for laser-scanning and developers respectively having Y, M, C and Bk toner located next to each other, it is possible to print a color image, produced by superimposing each of the color components, on a sheet of print paper 1028.
In order to express density of a print image (in a case of a color image, density of each color component), the amount of light corresponding to the density of an image to be printed is irradiated as described above.
In general, a pulsewidth modulation (PWM) system is known as effective means to control the amount of light irradiation. FIG. 13 shows an example of a construction which realizes a PWM system.
In FIG. 13, reference numeral 1 denotes a digital data output unit which converts printing data (multivalued image data) from an external device, such as a computer or the like, to a multivalued data string for each scanning line. Reference numeral 2 denotes a lookup table which inputs the multivalued data outputted from the digital data output unit 1, executes table conversion corresponding to predetermined function processing, and outputs multivalued data. Reference numeral 3 denotes a D/A converter which inputs the multivalued data from the lookup table 2, and outputs a corresponding analog voltage signal.
Reference numeral 5 denotes a horizontal synchronization signal generator based on the signal BD mentioned above; 6, a reference frequency generator; 7, a timing signal generator which generates a timing signal for each component of the apparatus; and 8, a triangular wave generator which generates an analog triangular wave in accordance with the timing signal (clock signal) supplied by the timing signal generator 7.
Reference numeral 4 denotes a comparator which compares the analog signal based on image data (output signal of the D/A converter 3) with the triangular wave outputted by the triangular wave generator 7, and outputs the comparison result as a logical high/low pulse signal. Accordingly, the comparator 4 outputs a signal having a pulsewidth corresponding to a pixel value. Reference numeral 8 denotes a raster scanning print engine, having a construction for driving a laser device in accordance with a pulsewidth of a pulsewidth modulation signal as described above with reference to FIG. 11. Therefore, in one pixel cycle, light emission time of a laser beam is determined based on a pixel value. In this manner, the amount of light emission can be controlled.
The above-described construction realizes an image modulator, in which inputted multivalued image data is subjected to xcex3 conversion with an appropriate function and subjected to pulsewidth modulation with a weight corresponding to the result of the xcex3 conversion.
Note as the aforementioned comparator, although an analog comparator is employed, by employing a digital comparator and generating a digital triangular wave, a digital pulsewidth modulator which directly compares the xcex3-converted multivalued image data with the digital triangular wave can be constructed. In the case of employing such digital system, characteristic variations due to a temperature drift and variations of components, which are problematic in the analog method, can generally be made small.
However, in the case of employing the above-described digital pulsewidth modulator which adopts a triangular wave comparison method, there are disadvantages, such as a necessity of a fairly high-speed counter and a high-speed comparator, as well as an enlarged size of the circuit.
In view of this, recently proposed is an image modulator in which an output pattern of a pixel is expressed in advance by a string of binary data representing light emission/non light emission, plural types of the output patterns are set in advance in a digital circuit for the number corresponding to values of inputted image tones, then upon inputting multivalued image data, the output pattern is selected and read out, and the read binary data strings are serially outputted.
For instance, assuming a case of inputting 4-bit input data (16 tones), 16 bits are prepared as pattern data. For input data xe2x80x9c3xe2x80x9d, a pattern having xe2x80x9c1xe2x80x9d for the first three bits and xe2x80x9c0xe2x80x9d for the subsequent thirteen bits is stored. When this data (value 3) is inputted, 1110000000000000B (B indicates binary data) is sequentially outputted, thereby generating a signal having a corresponding pulsewidth.
However, the above technique has the following problems.
Because the number of bits in a light emission pattern is only 2n (or (2n)xe2x88x921) while the number of bits of input data is n, the light emission pattern is determined practically on a one-to-one basis with the inputted image data. Therefore, it is impossible to correct variations in input-output functions, due to the output driving pattern corresponding to each input data and unevenness in electric and optical characteristics of a laser driver and laser device that follow the output driving pattern.
In a case where a block in which the output light emission pattern is set is fixedly constructed with logical hardware, because the light emission pattern is determined on a one-to-one basis with inputted image data, it is impossible to correct variations in input-output functions, due to the output driving pattern corresponding to each input data and unevenness in electric and optical characteristics of a laser driver and laser device that follow the output driving pattern.
If the aforementioned variations are individually measured in advance in a stage of apparatus assembling adjustment and stored in ROM constituting the block where the output light emission pattern is set, the variations of initial characteristics can be corrected individually. However, variations caused by environmental changes or variations generated over time cannot be corrected. In a case of adopting a RAM system, values are initialized from the system side when starting up the apparatus. However, since this is only to rewrite set data held by the system, the problem is not solved.
The present invention has been proposed in view of the conventional problems, and has as its object to provide a pulsewidth modulation apparatus, which enables appropriate pulsewidth modulation in accordance with variations of a device or a driving system in a case where pulsewidth modulation is performed with digital data patterns, and to provide a printer utilizing the pulsewidth modulation apparatus, and a control method thereof.
In order to solve the above problems, the pulsewidth modulation apparatus according to the present invention has, for instance, the following configuration. More specifically, a pulsewidth modulation apparatus for inputting multivalued pixel data expressed in a plurality of bits, generating a signal having a pulsewidth corresponding to a value of the multivalued pixel data, and outputting the generated signal to a predetermined print engine, comprises storage means arranged to store a digital pattern in an address position corresponding to the value of each multivalued pixel data in order to generate a binary-valued pattern corresponding to the value of the multivalued pixel data; and output means arranged to access the storage means by using inputted multivalued pixel data as an address to read a corresponding binary-valued pattern, and output a pulsewidth signal corresponding to a state of bits of the pattern, wherein the pattern stored in the storage means has a number of bits larger than a number that can be expressed by a number of bits of the inputted multivalued pixel data, and the pattern is rewritable.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.